The present invention relates to an integrated circuit switching voltage regulator circuit having multi-function terminals. More particularly, the present invention relates to an integrated circuit for use in implementing a switching voltage regulator circuit, the integrated circuit requiring only five terminals, operable in both feedback and isolated flyback mode, and including a power switching element, a driver network, and control circuitry which sets the duty cycle of the switching element.
The function of a voltage regulator is to provide a predetermined and substantially constant output voltage level from an unregulated input voltage. Two types of voltage regulators are commonly used today: linear regulators and switching regulators.
A linear regulator controls output voltage by controlling the voltage drop across a power transistor which is connected in series with a load. The power transistor is operated in its linear region and conducts current continuously.
A switching regulator controls output voltage by using a power transistor as a switch to provide a pulsed flow of current to a network of inductive and capacitive energy storage elements which smooth the switched current pulses into a continuous and regulated output voltage. The power transistor is operated either in a cutoff or saturated state at a duty cycle required by the voltage differential between the input and output voltages. Varying the duty cycle varies the regulated output voltage of the switching regulator.
The duty cycle of a switching regulator is controlled by monitoring output voltage or current through the switch. The latter type of switching regulator is known as a current-mode switching regulator, and is easier to frequency stabilize and has better response to transients than does a switching regulator in which the duty cycle is controlled directly by output voltage.
Switching regulators have at least two advantages over linear regulators. First, switching regulators typically operate with greater efficiency than linear regulators, a particularly important factor in high current regulators. Second, switching regulators are more versatile than linear regulators. Switching regulators can provide output voltages which are less than, greater than, or of opposite polarity to the input voltage, depending on the mode of operation of the switching regulator, whereas linear regulators can only provide output voltages which are less than the input voltage.
Further, switching regulators can be configured to drive current through the primary winding of a transformer, the secondary winding of which simultaneously provides current to the load. The transformer provides current gain, the amount of which is determined by the turns ratio of the transformer. Multiple outputs are possible, each output typically requiring two steering diodes, an inductor and a capacitor. Alternatively, the transformer may be configured such that current provided to the primary winding of the transformer by the regulator switch is stored as energy in the primary winding and is transferred to the secondary winding only after the switch driving the primary winding is opened. This configuration, known as flyback operation, allows multiple regulated output voltages and requires only one steering diode and one capacitor for each output.
In either of the above transformer configurations, the isolation provided by the transformer between input and output circuits is limited by the need to regulate the output voltage by sensing the output voltage of the regulator circuit and providing a feedback voltage signal to the control circuitry of the circuit. The output circuit driven by the secondary winding of the transformer thus remains electrically connected to the input circuit driving the primary winding. Voltage regulator configurations which sense output voltage of the circuit for use as a feedback signal are referred to herein as normal feedback mode regulators. Another configuration, known as an isolated flyback mode regulator, allows a transformer secondary winding to be totally isolated from the input circuit connected to the primary winding by regulating the peak voltage developed across the primary winding when the secondary winding provides current to the output circuit.
Switching regulators, although more flexible than linear regulators in circuit applications, are typically more complex than linear regulators. Although several integrated circuits in the past have been commercially available for implementing the control, driver and power switch functions of switching regulators, switching regulators utilizing such integrated circuits have required substantial engineering expertise and numerous discrete components to make them operational. Also, integrated circuits heretofore available typically required 8-14 terminals for connection to external discrete components, and could not be configured into a very low current (shutdown) mode. This quantity of terminals prevented such integrated circuits from being packaged in low-cost power transistor packages such as the conventional 5-pin TO-3 type metal can or the TO-220 type molded plastic packages, and thus limited the power handling capability of the integrated circuit. Further, heretofore available integrated circuits for use in implementing switching voltage regulators have not been capable of use both in normal feedback mode switching regulator circuits and isolated flyback mode regulator circuits.
In view of the foregoing, it would be desirable to be able to provide a switching voltage regulator circuit which is simple to implement and which is capable of versatile and efficient operation.
It would further be desirable to be able to provide a switching regulator circuit, having a very low current sleep mode which can be implemented as an integrated circuit which includes the power switch, and which can be packaged in a conventional TO-3 or TO-220 power transistor package.
It would also be desirable to be able to provide an integrated circuit which can be utilized to implement both normal feedback mode and isolated flyback mode regulator circuit topologies.